The emergence of liquid-crystal screen technology offers an excellent outlet for video projection techniques. The light emitted by an arc lamp is modulated by a liquid-crystal cell. The image formed by the liquid-crystal display device is projected by an optical system onto a screen. The so-called AM-TFT TNLCD technique (that is to say twisted-nematic liquid-crystal display controlled by an active matrix of thin-film transistors) is regarded as essential for liquid-crystal screens, each picture element (pixel) being controlled by a transistor. The light incident on the liquid-crystal screen must be linearly polarized. The major drawback of this technique is its low efficiency.
In fact, 1 to 2% of the light reaches the screen. Three main parameters limit this efficiency, namely:
1--more than 50% of the light is lost (-60%) since the light coming from the lamp is not polarized; PA1 2--the filling factor of the cell is limited, especially for the definition of a large image and the small diameter of the liquid-crystal modulator. The filling factor or OAR (Open Aperture Ratio) is about 50%; PA1 3--since the lamp is not small, the illumination of the small diameter of the LCD liquid-crystal display (the light beam being defined by the solid angle adapted to the contrast of the LCD display) decreases the light efficiency. Screens of 16:9 format have a light efficiency of less than 40%. PA1 E(mm.sup.2.sr)=S(circular area).times..OMEGA. PA1 where: PA1 2E=(2S).times..OMEGA., which is not acceptable because of homogeneity and space problems; PA1 2E=S.times.(2.OMEGA.)--the current techniques demonstrate that many systems do not achieve this minimum value (i.e. 2.OMEGA.), but higher than this. PA1 2f.times.tan (.beta..sub.glass).ltoreq.active area PA1 where:
Other factors cause attenuation, such as colour rendition, white balancing and Fresnel losses.
Many solutions have been proposed to improve the light efficiency of these projection systems. Some solutions propose the conversion of the second polarization (see, for example, the document "Large Aperture Polarized Light Source and Novel Liquid Crystal Display Operating Modes" S. V. Belayev, M. Schadt, M. I. Bamik, J. Funufschilling, N. V. Malimoneko and K. Schmitt. Japanese Journal of Applied Physics, Vol. 29., April 1990, pp. L634-L637) in the illumination box, and others recommend the use of microlenses intended to concentrate the light in the active area of the pixels in the screen (see for example the document "Brightness Enhancement of an LCD Projection by Planar Microlens Array", H. Hamada, F. Funada, M. Hijikigawa and K. Awane, SID 92 DIGEST, pp. 269-272).
The invention relates to a high-performance polarizing converter which can be combined with conventional microlenses so as to obtain a high-efficiency projector, characterized by a performance which is superior, possibly by up to a factor of 3, compared to a conventional system.
In order to illustrate the various improvements made to the proposed systems, we have worked on the basis of the useful geometrical extent or analysis of the extent.
The value of the extent of a light beam through a surface S is the product of the area of the said surface multiplied by the solid angle defining the light beam:
.OMEGA.=2.pi.1-cos (.beta.)!, PA2 .beta. being the illumination half-aperture. PA2 the active surface area of the pixel is equal to the pixel area less the area of the black matrix (masking matrix), i.e. size of the pixel--black matrix; PA2 f is the thickness of the liquid-crystal screen; and PA2 .beta..sub.glass is the illumination half-angle within the glass of the LCD display.
The lamp used in the liquid-crystal projection display system has a spatial extension (that is to say a non-zero spatial extension); it may be characterized by its extent: E.sub.lamp or Flux (Flux=E (Extent).times.(L (Luminance), if L is constant).
Moreover, the contrast of the LCD liquid-crystal display is largely dependent on the illumination aperture. If .beta.&lt;.+-.10 deg., the contrast will be always acceptable for projection. Thus, .OMEGA. is limited to a value .OMEGA.1. The said limitation is also associated with the objective lens used for the projection.
Furthermore, liquid-crystal screens having quite a small diameter have been chosen so as to decrease the cost of the said systems and their optical components, resulting in a small illumination area S1.
If the product of S1.times..OMEGA.1, that is to say E1, is less than E.sub.lamp, the light efficiency will be poor and equal to the ratio E1/E.sub.lamp.
If E1 is greater than E.sub.lamp, that is to say the light efficiency will be equal to 100%.
If E1 is equal to E.sub.lamp, the system will be satisfactorily optimized.
The polarizing conversion system doubles the value of the extent E since the light from the two polarization components is spatially split into two directions.
After having passed through the polarization splitter, the light E becomes 2E. At this stage, 2 cases may be considered:
Analysis of the extent may be applied to the method of focusing the light onto the pixels in the liquid-crystal screen. For conventional focusing using a matrix of spherical microlenses--one microlens for each pixel for 100% focusing--hence the need to obtain a filling factor of 100% and no longer 50% (or less); the reason for this is that the focusing area is less than or equal to the active area of the pixel, that is to say:
Knowing the parameters of the system--f, .beta..sub.glass, size and dimensions of the LCD display--it is thus possible to define the quantity of light passing through the screen after focusing:
E.sub.focusing =S (circular area at the periphery of the LCD display).times.2.pi.1-cos (n..beta..sub.glass)!,
where n is the refractive index of the glass of the LCD display.
As mentioned previously, the total efficiency is the ratio of E.sub.focusing to E.sub.lamp.
Conventional illumination systems provided with polarizing or focusing-based conversion devices cannot actually be used because of the extension of the value of extent, characteristic of the polarizing converter for example, thereby cancelling out the advantage of the focusing, even if the lamp has a restricted geometrical extent. The invention solves this problem and gives a gain of greater than 3 in the light flux.
Furthermore, extension of the solid angle .OMEGA. may be performed in various ways--either in the direction of the orifice or in another direction (horizontal or vertical with respect to the distribution of the isocontrast of the LCD display). Extension in the correct direction for the contrast of the screen is possible in this case.